1. Field of the Invention
This invention relates generally to rotor devices and, more particularly to screw rotors.
2. Description of Related Art
Screw rotors are generally known to be used in compressors, expanders, and pumps. For each of these applications, a pair of screw rotors have helical threads and grooves that intermesh with each other in a housing. For an expander, a pressurized gaseous working fluid enters the rotors, expands into the volume as work is taken out from at least one of the rotors, and is discharged at a lower pressure. For a compressor, work is put into at least one of the rotors to compress the gaseous working fluid. Similarly, for a pump, work is put into at least one of the rotors to pump the liquid. The working fluid, either gas or liquid, enters through an inlet in the housing, is positively displaced within the housing as the rotors counter-rotate, and exits through an outlet in the housing.
The rotor profiles define sealing surfaces between the rotors themselves between the rotors and the housing, thereby sealing a volume for the working fluid in the housing. The profiles are traditionally designed to reduce leakage between the sealing surfaces, and special attention is given to the interface between the rotors where the threads and grooves of one rotor respectively intermesh with the grooves and threads of the other rotor. The meshing interface between rotors must be designed such that the threads do not lock-up in the grooves, and this has typically resulted in profile designs similar to gears.
However, a gear tooth is primarily designed for strength and to prevent lock-up as teeth mesh with each other and are not necessarily optimum for the circumferential sealing of rotors within a housing. As discussed above, threads must provide seals between the rotors and the walls of the housing and between the rotors themselves, and there is a transition from sealing around the circumference of the housing to sealing between the rotors. In this transition, a gap is formed between the meshing threads and the housing, causing leaks of the working fluid through the gap in the sealing surfaces and resulting in less efficiency in the rotor system.
Some arcuate profile designs improve the seal between rotors by minimizing the gap in this transition region. Single thread profiles can result in imbalances in the rotors when rotated at high speeds and multiple thread profiles allow for leaks between the positive displacement flow regions bounded by the multiple threads. The leaks between multiple threads in these rotors can be significant in prior art designs because the rotor length extends beyond a single pitch of the threads. However, many of the prior art thread designs use multiple pitch threads. Additionally, these designs are based on multiple curves in a lengthwise cross-section. Multiple curves impose manufacturing constraints that adversely impact the ability to manufacture the rotors and to maintain close tolerances between the rotors.
It is in view of the above problems that the present invention was developed. The invention features a screw rotor device having an identical number of threads (N), a buttress thread profile with a diagonal line, and a length that is either approximately equal to or less than a single pitch of the threads. Another feature of the invention is the cross-sectional shape of the rotors. In particular, for twin rotors, the cross-sectional shape is identical. The features of the invention result in an advantage of improved efficiency and manufacturability of the screw rotor device.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.